The present invention relates to the field of the controlled deposition of molecules on surfaces on a nanometric scale.
The optical manipulation of the atoms constituting an atomic beam has been widely studied over the past few years. It has been shown, for instance in the article xe2x80x9cCalculation of Atomic Positions in Nanometer-scale Direct-write Optical Lithography with an Optical Standing Wavexe2x80x9d, by K. K. Berggren et al., published in Journal of the Optical Society of America B, Vol. 11, pp. 1166-1176 (1994), and in the references thereto, that an atomic beam can be focused to sub-micron scale dimensions by using the dipole forces exerted on the atoms by an electromagnetic field, such as that present in a standing light wave. One possible application of this phenomenon is in direct-write atomic nanolithography, which offers the possibility of microfabrication applications in the microelectronic industry, at resolutions well below the wavelength of ultra-violet light, as currently used.
In U.S. Pat. No. 5,360,764 to R. J. Celotta and J. J. McClelland, hereby incorporated by reference, there is described the use of a combination of laser cooling techniques and periodic standing wave electromagnetic fields to enable the focusing of atoms and their subsequent deposition on a substrate, on a nanometric scale. However, the technique described therein has a number of disadvantages; (i) it is limited to the controlled deposition of atoms, and there are many practical chemical processes, where the presence of molecules rather than atoms is preferable (ii) it requires extensive laser cooling to narrow the lines sufficiently to provide focusing with good resolution, and (iii) it is limited to the formation of periodic structures on the surface. The technique is thus both limited in the type of materials that can be deposited to atomic species, and also in the range of positions capable of deposition. Furthermore, because of the transverse laser cooling required to provide good collimation of the beam in the longitudinal direction, the process is not simple to apply, involving the use of frequency shifted optical pumping and trapping laser beams.
A method of depositing molecules is also described in the Celotta et al. patent, whereby more than one atomic species are concurrently evaporated onto the desired substrate surface. One of the atomic species is focused into the desired pattern on the substrate by selecting the conditions to ensure that it is in resonance with the applied laser field, and the other, or others, are applied uniformly. At the positions of focus, the two atomic species react chemically to form the desired molecular deposit. Using this scheme, a method for the formation of an array of spots of CrO2 is described therein. It is evident that this method for the deposition of molecules is complicated to perform, and it may prove difficult to achieve good stoichiometry.
Methods of direct manipulation of the molecules of a molecular beam by means of optical focusing, analogous to the methods described above of atomic beam manipulation, have not yet proved particularly successful. The concepts associated with molecular manipulation have indeed been considered for several years, such as is described, for instance, in the article entitled xe2x80x9cDeflection of Neutral Molecules Using a Nonresonant Dipole Forcexe2x80x9d by H. Stapelfeldt et al., published in Physical Review Letters, Vol. 79, pp.2787-2788, 1997, and in the earlier references cited therein. The comparative lack of success is due, in large part, to the fact that the optical cooling and trapping techniques developed for atoms are not readily applicable to molecules. There therefore exists an important need for a method for the controlled deposition of molecules on surfaces on a nanometric scale.
The disclosures of the each of the publications mentioned in this section, and of those in the other sections of this specification, are hereby incorporated by reference, each in its entirety.
The present invention seeks to provide a new method and apparatus for the optical focusing of molecular beams, such that the molecules can be deposited in aperiodic structures, with resolutions of down to 10-15 nanometers. The ability to deposit molecules on surfaces at a nanometric scale has important applications in the semiconductor industry for the purposes of direct deposition etching and for other lithographic processes. The method requires the use only of mechanical cooling, such as is provided by expansion of the molecular beam through a supersonic nozzle, thereby considerably simplifying the process in comparison with the optical collimation processes needed for use in atomic beam focusing. The nature of the pattern formed, including the position and width of the component parts of the pattern, are altered by varying a number of parameters associated with the beam preparation and with the electromagnetic fields to which the beam is subjected.
A beam of molecules, aimed at the surface on which the deposition is required, is sent through a skimmer to minimize velocity components perpendicular to the direction of the beam. It is then subjected to an electromagnetic field such as may be provided by one or more laser beams, either pulsed or CW, which prepares a linear superposition of bound states, primarily through a two photon absorption process. This operation is another application of the process of coherent control, which has been developed recently to affect atomic and molecular processes by means of quantum interference. Up to now, coherent control has been used to control the outcome of unimolecular processes such as photodissociation, and more recently, collisional and scattering processes. Details of the theory and some applications of the technique of coherent control are contained in the articles xe2x80x9cPolarization Control of Branching Ratios in Photodissociationxe2x80x9d by C. Asaro, P. Brumer and M. Shapiro, published in Physical Review Letters, Vol. 60, pp. 1634-1637 (1988) and in xe2x80x9cCoherent Control of Reactive Scatteringxe2x80x9d by A. Abrashkevich, M. Shapiro and P. Brumer, published in Physical Review Letters, Vol. 81, pp. 3789-3792 (1998), and in the many references cited therein.
In U.S. Pat. No. 5,256,849, to M. O. Scully, there is described a method of increasing the refractive index of a material by means of the creation of superpositions of states therein, by means of coherent control of the atomic levels of the material. Alteration of the refractive index of a material is operative to affect the motion of light through the material. The use of coherent control in the present invention, unlike any of the methods described in the prior art, is operative to affect the motion of the molecules themselves, by means of optical focusing.
The prepared molecular beam then passes through two or more standing electromagnetic fields directed parallel to the surface, which too may be produced by means of interacting laser beams. By varying the characteristics of the laser beams, the molecular properties, the distance of the stationary fields from the surface, and the properties of the stationary electromagnetic fields, the nature of the pattern deposited on the surface can be controlled, including the position, intensity and resolution of the component parts of the pattern. In general, the pattern displays a large background with several relatively low intense peaks when there is no molecular coherence, whereas the peaks become intense and the background weak when the molecular coherence is introduced. The position of the peaks is controlled primarily by the optical coherence, whereas, the peak intensity is controlled by the molecular coherence of the beam.
There is thus provided in accordance with a preferred embodiment of the present invention, a method of depositing molecules in a predetermined pattern onto a surface by means of coherently controlled optical focusing of a beam of the molecules.
There is further provided in accordance with yet another preferred embodiment of the present invention, a method as described above and consisting of the steps of providing a collimated beam of molecules to be deposited, directing the beam through a first electromagnetic field, typically produced by a laser beam, operative to produce a superposition of bound states of the molecules, and thereafter directing the beam through a second electromagnetic field, typically produced by two or more standing waves, such that the molecules are focused onto the surface in the predetermined pattern.
In accordance with still another preferred embodiment of the present invention, there is provided a method as described above and also consisting of the step of cooling the beam of molecules before production of the superposition of bound states, the cooling being optionally effected by either a mechanical or a laser cooling process.
There is further provided in accordance with still another preferred embodiment of the present invention, a method as described above and wherein the superposition of bound states of the molecules is formed by means of a two-photon absorption process.
In accordance with a further preferred embodiment of the present invention, there is also provided a method as described above and wherein the mechanical cooling process is effected by expansion of the beam through a supersonic nozzle.
There is provided in accordance with yet a further preferred embodiment of the present invention, a method and wherein the first laser is either a CW or a pulsed laser.
There is even further provided in accordance with a preferred embodiment of the present invention, a method as described above and wherein the standing waves are formed by one or more laser beams.
Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided a method as described above and wherein the predetermined pattern is aperiodic and may be determined at least by the parameters of the first electromagnetic field and by the parameters of the second electromagnetic field.
There is also provided in accordance with a further preferred embodiment of the present invention, a method as described above and also consisting of the step of directing the beam through a third electromagnetic field, arranged approximately orthogonally to the second electromagnetic field, and in effectively the same common plane, such that the molecules are focused onto the surface in a predetermined array pattern, which could have a resolution of less than 50 nanometers.
In accordance with yet another preferred embodiment of the present invention, there is provided a method of depositing molecules in a predetermined pattern onto a surface as described above, and wherein the molecules are operative to perform applications such as nanolithography, micro-etching, the writing of information on a storage medium, the formation of photolithographic masks, the production of doped regions within the surface, the production of high profile tip structures on the surface, or the production of optical grating structures on the surface.
There is further provided in accordance with yet another preferred embodiment of the present invention, a system for the deposition of molecules in a predetermined pattern onto a surface by means of coherently controlled optical focusing of a beam of the molecules.
In accordance with still another preferred embodiment of the present invention, there is provided a system for the deposition of molecules as described above, and consisting of a source emitting a beam of the molecules, a collimator for minimizing the transverse velocity components of the molecules of the beam, a first electromagnetic field through which the beam is directed, operative to produce a superposition of bound states of the molecules, and a second electromagnetic field, through which the beam is thereafter directed, such that the molecules are focused onto the surface in the predetermined pattern.
There is further provided in accordance with still another preferred embodiment of the present invention, a system for the deposition of molecules as described above and also consisting of a cooler for cooling the beam of molecules before production of the superposition of bound states, the cooler utilizing a mechanical cooling process or a laser cooling process.
In accordance with a further preferred embodiment of the present invention, there is also provided a system for the deposition of molecules as described above, and wherein the mechanical cooling process consists of the expansion of the beam through a supersonic nozzle.
There is provided in accordance with yet a further preferred embodiment of the present invention, a system for the deposition of molecules as described above and wherein the first electromagnetic field is formed by at least one first laser beam.
There is even further provided in accordance with a preferred embodiment of the present invention, a system for the deposition of molecules as described above and wherein the second electromagnetic field consists of at least two standing waves formed by laser beams.